In colder climates, where aggregate is often stored below freezing temperatures, it is generally known to heat aggregate to ensure that water does not freeze in contact with the aggregate when mixing with concrete.
A known type of aggregate heater is an air heated bin in which a bin structure having an open top end for loading and a hopper at a bottom end for discharge further includes a plurality of fluid inlets at different elevations for diffusing the hot air through the aggregate in the bin. One example of a bin heater system of this arrangement is described in U.S. Pat. No. 3,659,583 by Martin.
In a typical aggregate condition bin, the level of material in the bin is highly variable since the feeding of material to the bin and the consumption of the material from the bin are not constant and equal rates. When using known heater systems of the type described above, air follows the path of least resistance through the aggregate. Accordingly, if the variable level of aggregate material within the bin falls below the uppermost fluid inlets, the heated air primarily escapes to atmosphere through the exposed fluid inlets rather than being used effectively to heat the aggregate.
It is also desirable to keep the bin close to being full for optimal use of the heated air, but few reliable means are known for monitoring the height of the entire top surface of the aggregate in the bin.